Many applications call for the operation of alternating current (AC) discharge loads such as discharge lamps, including ultraviolet (UV) discharge lamps. For example, UV lamps are used for curing inks in printing systems. Many other uses for UV lamps are popular, representative examples of which include curing furniture varnish or heat-sensitive substrates, decontaminating food substances, sterilizing medical equipment or contact surfaces, optically pumping solid state lasers, electrically neutralizing surfaces, inducing skin tanning, and passing through fluorescent coatings to provide visible illumination. Additional uses for discharge lamps in other wavelengths are also popular, such as visible wavelength discharge lamps for providing illumination.
It is often desired for multiple discharge lamps to operate together as part of a system. For instance, in printing operations, it is common for separate discharge lamps to be used to cure each color ink that is applied, or for each step in a printing process.
Discharge lamps must be supplied with electrical power. Electrical power is normally derived from a standard AC utility source, which typically drives the primary sides of ballast transformers, the secondary sides of which provide electricity to the lamps.
A gas discharge lamp applies this electricity to the gas or vapor within a lamp. Several varieties of gas or vapor are used in gas discharge lamps. Mercury vapor is a popular choice; other gas discharge lamps are based on gallium, halogen, metal halide, xenon, sodium, or other varieties. Whatever particular chemistry is used, the electricity ionizes the gas within the lamp, so that when electrons recombine with ions, light is emitted. This discharge light is alternately described as an arc, a glow, or a corona.
For a gas molecule to ionize, a minimum threshold electric field must be applied to it. A lesser field will only polarize gas molecules without causing ionization. So, an ignition voltage is typically required for a discharge lamp to achieve ionization of the gas molecules.
Once ionization begins, it initially drives a positive feedback chain reaction as the initially freed electrons collide with other polarized molecules close to the ionization energy and provide the extra energy needed to ionize. As the populations of ionized molecules and free electrons rise, the rate of recombination also rises, until an equilibrium is reached where the rate of new ionizations is equal to the rate of recombinations. A discharge load goes from the initial equilibrium with no current, through the unstable ignition transition with negative resistance, to the new operating equilibrium.
It is typically desirable to compensate for the negative resistance of the discharge load during the ignition transition, and to provide a lower voltage than the ignition voltage when the ionization equilibrium has been achieved. An enhanced level of current is often used for warm-up, while a lower run level of current is required to maintain normal operation.
A discharge lamp will therefore have a rated operating current and a rated operating voltage, while the actual values of current and voltage through the lamp outside of normal operation, such as during ignition and warm-up, may vary considerably. Discharge lamps come in a wide range of sizes, and a correspondingly wide range of current, voltage, and power ratings. The voltage and power ratings on many lamps are considerably high.
The current, voltage, and power characteristics over time of the electrical supply must therefore be controlled within acceptable tolerances. The voltage provided to such lamps is typically in alternating current (AC) form. Allowing any net direct current through a discharge lamp often causes undesirable effects, such as gas migration and accumulation on the lamp electrodes, and saturation of an associated ballast.
The ballast is intended to provide a discharge lamp with a supply of electricity in a form that should remain controlled to have these proper characteristics of voltage and current. Traditionally, these are magnetic ballasts, that include end stage transformers placed in connection with the lamps, and banks of high-voltage capacitors.
Each ballast must power two interfaces. A ballast must have a utility interface and a lamp or load interface. A voltage is provided by the utility, and the ballast will draw a current from this voltage. The power drawn from the utility is supplied, typically without substantial loss, via an output interface to the lamp.
Typical gas discharge lamps must have a controlled current supplied to them because they are substantially constant voltage loads. A function of the ballast is to convert the power supplied at a substantially constant voltage from the utility, to a controlled current and substantially constant voltage which it delivers to the lamp. Although the utility voltage and lamp voltages are alternating current, they are typically substantially constant in the sense that their root mean square (RMS) value is substantially constant, as is familiar to those skilled in the art.
However, these traditional solutions have substantial drawbacks. For example, a traditional ballast may have only one set amount of power it can provide to its lamp, or at best only two or three options for power settings. For another example, a traditional ballast may have only a single voltage setting that is tailor-made for a specific lamp. This means a multi-lamp system will impose separate maintenance and replacement requirements for each of several different ballasts. As another example, traditional ballasts often provide a substantially inaccurate or variable current, with typical inaccuracy of up to 20% or more. As another example, traditional ballasts are often electrically inefficient and convert a significant fraction of current into waste heat, causing the ballasts to operate at high temperature, often leading to additional problems. As another example, traditional ballasts are often bulky, heavy, inconvenient, and expensive. To illustrate, a typical ultraviolet discharge lamp used for curing inks in a printing operation may be twelve feet long, and be supplied by a transformer ballast weighing 700 pounds.
Traditional ballasts also have the disadvantage of inflexibility, in that each ballast must interface directly between a utility voltage supply and a load. The load requires a controlled current for substantially constant voltage. Each ballast must supply sufficient power from the utility supply to cover the peak demand of the corresponding discharge load. In a system of many loads, the total power can be substantial, and the direct and indirect costs of the several individual ballasts are similarly substantial. The greater the system demand for electrical power, the greater the initial capital costs and the ongoing maintenance and power costs. A system of many lamps, each with a corresponding ballast with individual utility interface and lamp interface, also has significant complexity.
For example, a typical discharge lamp system in a printing operation might have nine discharge lamps, each drawing a peak power of 15 kilowatts. In a typical ballast system, each of these lamps would be used with a corresponding ballast having a utility interface function rated for 15 kilowatts, and a discharge lamp interface function rated for 15 kilowatts. Each ballast must be capable of operating for long periods of time, such as hours or days, at 15 kilowatts. The total system therefore has not only a sum of 135 kilowatts of lamp interface capacity, but also a sum of 135 kilowatts of utility interface capacity.
In typical operation, the several lamps tend to draw different amounts of power at different times, so that typically no more than a few lamps draw their peak amount of power at one time. The average power drawn by the lamps might typically be 50 kilowatts with regular relative peaks of around 100 kilowatts, with the absolute peak of 135 kilowatts only reached occasionally and briefly. Much of the ballast capacity, installed and maintained with considerable expense and complexity, therefore spends much of its time idle.
A new solution is therefore highly desired for the problem of delivering electrical power to discharge lamp ballasts. It is further desired that such a solution may introduce greater flexibility and efficiency to fulfilling the power supply requirements of a multiple lamp ballast system, with the ultimate goal of reducing initial and ongoing costs.